The present invention relates to a single screw extruder comprising a barrier screw and a barrel in which the barrier screw is rotatably mounted and which comprises at least a feed zone longitudinal section and a melting zone longitudinal section. The invention further relates to a method for extruding plastic material using such a single screw extruder.
A single screw extruder having a barrier screw is for example disclosed in the paper “Zylinder glatt oder genutet”, Maschinenmarkt, Würzburg 95 (1989) 48, pages 40–43. In this paper different concepts of barrier screws are presented which are used in extruders having smooth or grooved feed areas. The barrels in the area of the melting zone longitudinal sections are formed in a plane and smooth manner for a better separation of melt and solid matter.
A screw housing for an extruder is known from DE 25 14 307, which comprises at least one groove extending across the whole length. This groove serves to avoid a “screw slip”, that is a rotation of the screw without a transport of material in the barrel. The screw shown in this document is not a barrier screw.
Generally single screw extruders are known. They typically comprise an extruder plastification barrel in which a screw is rotatably mounted. A powdered or granulated initial material (with stabilizers, slip additives, if required thermoplastic materials being added with fillers and colorings) is fed at one end of the barrel and is transported by the rotating screw through the barrel to a so-called demoulding tool or matrix located at the opposite end. At first, the initial material is conveyed or transported through the so-called feed zone and is thereby compressed. This feed zone is followed by a melting zone and plastification zone, respectively, in which the initial material is melted by friction at the barrel inner surface. The melting process may—particularly when starting the extruder—be supported by an outer heating of the barrel via (mostly electrical) heating elements. Depending on the application the melting zone is followed by a homogenizing zone and a demoulding zone in which the plastified material is prepared for further processing.
In recent years, single screw extruders being based on the so-called barrier screw concept have gained acceptance to an increasing extent. In a so-called barrier screw, the screw channel is divided in a solid matter channel and a melt channel by way of a further side bar. Compared with the primary side bar, being known from conventional screws, the smaller formed barrier side bar allows a transverse flow of melt from the solid matter channel into the melt channel. The cross-section of the melt channel increases in a downstream direction whereas the cross-section of the solid matter channel continuously decreases in the downstream direction as to maintain the desired conveying effect of the screw. Due to the so-called barrier zone particularly the heat transfer from the barrel and the screw surface to the granulate not yet melted is improved.
Although this barrier screw concept has been proven practical for many applications still remains it the need to increase the performance of the single screw extruder with barrier screw while keeping the necessary structural measures as low as possible.